1. Field of the Invention
The present invention relates to an aligning apparatus, and an aligning method for aligning a plurality of medical images. The present invention also relates to a program for causing a computer to function as the aligning apparatus.
2. Description of the Related Art
When performing a comparative interpretation of radiation images of the same region of the same subject, one was obtained recently and the other was obtained, for example, a year ago, it has been known that it is effective to generate a differential image based on the difference between the two images using the temporal subtraction technique. When generating a differential image based on the difference between the two images using the temporal subtraction technique, it is important to accurately align the subjects in the images to observe the difference between the two images.
Recently, along with wider use of CT scanners and MRI machines, it is often the case in which diagnostic and treatment plans are developed by comparing tomographic images of the same subject. It is difficult to obtain the images with the subject having the same bodily posture and position if the images are obtained at different times, since the bodily postures and positions of the subject on the bed are not always the same. Further, when imaging the subject using a CT scanner or a MRI machine, the imaging conditions including slice thickness, slice pitch, image size, and the like may differ in each case. Therefore, image alignment has been conducted mostly by the doctors based on their knowledge and experience in the past. Consequently, the aligned position has differed slightly from doctor to doctor, which has prevented the image observation to be performed under the same conditions. Under these circumstances, an image aligning method having reproducibility without depending on the knowledge and experience of the doctors is proposed as described, for example, in Japanese Unexamined Patent Publication No. 10(1998)-137231. In the method, two images of a plurality of images are aligned based on the anatomical information such as bones or the like.
When diagnosing a respiratory function using images of a plurality of respiratory stages, it is difficult to directly align the image of maximum expiratory stage and that of the maximum inspiratory stage, since difference in the size of the lung fields is great due to three-dimensional movement of the ribs and diaphragm between the maximum expiratory stage and maximum inspiratory stage, and distances and directions also differ between the ribs and diaphragm. Consequently, a method in which the maximum expiratory stage and maximum inspiratory stage are gradually transformed into the intermediate stage is proposed as described, for example, in U.S. Patent Application Publication No. 20050025365.
Further, based on a three-dimensional image data which are the collection of a plurality of slice image data obtained by CT (Computer Tomography) or PET (Positron Emission Tomography), a cross-sectional image is often generated, or a three-dimensional image is displayed to find an affected area, or observe the state of the affected area for diagnosing the presence of a disease or the progress thereof. The three-dimensional image data include more data related to the subject compared with the conventional two-dimensional radiation image data, so that a more accurate diagnosis may be made. In addition, observation of images obtained by a plurality of different modalities allows to developing more accurate treatment plan.
The PET image is an image obtained after an isotope labeled agent is injected. In the PET, isotope labeled glucose is used as the agent (FDG). Cells use glucose as the source of energy. A cancer cell is more active than a normal cell, and takes up a larger amount of glucose as nutrition. Accordingly, it also absorbs a larger amount of FDG, and emits a larger amount of radiation compared with a normal cell. The amount of radiation is proportional to the amount of glucose absorbed by the tumor cell, i.e. activity of the cell, so that the PET may obtain an image reflecting the activity of the cancer cell area. In this way, the active cancerous lesion may be observed by the PET image. The PET image, however, has a low spatial resolution, causing the image to be blurred, so that it is difficult to correctly identify the position of the lesion using only the PET image. Consequently, treatment plan is usually developed using both a CT image, which allows accurate identification of the position and shape of the organs, and a PET image.
In the CT image, the positions and shapes of the organs appear clearly, while in the PET image, the shapes of the organs are blurred although an active cancerous lesion may be indicated. Thus, it is difficult to perform image alignment between the CT and PET images. Further, the alignment between CT and MRI images is also difficult, since no bones appear in the MRI image, although they appear in the CT image. As described above, different tissues appear in the images obtained by different modalities (clinical test equipment) Therefore, correct alignment of these images is difficult.
Consequently, an image aligning method that uses distinguishing shapes appearing in the images, such as lung fields or the like, when performing image alignment between the images obtained by different modalities is proposed as described, for example, Japan Journal of Medical Informatics, vol. 22, No. 6, pp. 706-707, 2002 (supplement of The 22nd Joint Conference on Medical Informatics).
Another method is also proposed as described, for example, in Japanese Unexamined Patent Publication No. 2002-248083, in which an image of a patient is obtained with a mask put on the patient. The mask has a protruding portion in which a marker is held. The marker appears in the image obtained by each modality, and a plurality of markers is provided, such as for CT/MRI, SPECT/CT, MRI/CT, and the like, since different markers appear in the images obtained by different modalities. When obtaining an image using each modality, the image is obtained with the mask having an appropriate marker for the modality in the protruding portion thereof is put on the patient. In this way, the data obtained by different modalities are aligned with reference to the marker.
The method disclosed in Japanese Unexamined Patent Publication No. 10(1998)-137231 is a method for performing alignment between two images. Thus, when performing alignment for three or more images, the alignment needs to be performed for each two images as shown in FIG. 10A. Accordingly, if the number of images is great, a great number of alignments are required.
Further, either method disclosed in Japanese Unexamined Patent Publication No. 10(1998)-137231, or U.S. Patent Application Publication No. 20050025365 is an image aligning method based on the assumption that the imaged region is substantially corresponding with each other. Therefore, if the imaged region differs significantly with each other, it is impossible to correctly align the images.
Still further, in the method disclosed in the non-patent document described above, it is necessary to know the effective tissue for the image alignment in advance for every combination of the modalities, and, what is more, the effective tissue for the image alignment is not always available. In addition, most of the tissues used for image alignment are soft tissues which may vary with the postural change of the subject, resulting in unfavorable alignment.
In the method disclosed in Japanese Unexamined Patent Publication No. 2002-248083, wearing the mask is burdensome for the patient. The aligning method using the marker is surely effective when images are obtained by a plurality of modalities at the same time frame. But, if the images are obtained, for example, by CT and PET at different time frames, it is difficult to have the subject wear the markers at the same position.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an aligning apparatus and an aligning method capable of accurately aligning images having different imaged regions, or images obtained by different modalities. It is a further object of the present invention to provide a program for causing a computer to function as the aligning apparatus.